Most copepods exhibit one of three kinds of body architecture: 1) six broad anterior trunk somites and five narrow posterior trunk somites of gymnopleans; 2) five broad anterior trunk somites and six narrow posterior trunk somites of many podopleans; or 3) four broad anterior trunk somites and seven narrow posterior trunk somites of thaumatopsylloids. A phylogenetic analysis using naupliar and post-naupliar characters, with Mystacocarida as the sister taxon of Copepoda, supports the hypothesis that the thaumatopsylloid architecture is the oldest. No narrow somite is transformed into a broad somite during post-naupliar development of thaumatopsylloids. Podopleans and gymnopleans begin their post-naupliar development with one trunk somite fewer than thaumatopsylloids. Podoplean architecture results when the anterior narrow somite of thaumatopsylloids is transformed to the posterior broad somite of podopleans during the first post-naupliar molt. Gymnoplean architecture, the youngest, results when the anterior narrow somite found on podopleans is transformed to the posterior broad somite during the second post-naupliar molt. These developmental transformations of body somites are assumed to explain the evolutionary origins of podoplean and gymnoplean body architectures.